Pre-charge sensing scheme for non-volatile memory (NVM)

ABSTRACT

The pipe effect can significantly degrade flash performance. A method to significantly reduce pipe current and (or neighbor current using a pre-charge sequence) is disclosed. A dedicated read order keeps the sensing node facing the section of the pipe which was pre-charged. The technique involves pre-charging several global bitlines (such as metal bitlines, or MBLs) and local bitlines (such as diffusion bitlines, or DBLs). The pre-charged global bitlines are selected according to a pre-defined table per each address. The selection of the global bitlines is done according to whether these global bitlines will interfere with the pipe during the next read cycle.

CROSS-REFERENCE(S) TO RELATED APPLICATION(S)

This application is a divisional application of U.S. patent applicationSer. No. 12/232,437 filed Sep. 17, 2008, which has matured into U.S.Pat. No. 8,098,525; and which claims benefit of U.S. Provisional PatentApplications 61/006,039 filed Dec. 17, 2007, and 60/972,840 filed Sep.17, 2007, all of which are hereby incorporated by reference in theirentirety.

TECHNICAL FIELD

The disclosure relates to techniques for operating semiconductor devicesand, more particularly, to operating non-volatile memory (NVM), such asfloating gate (FG) devices or charge-trapping devices such as nitrideread only memory (NROM), or other microelectronic cells or structures,such as reading (sensing the program state) of NVM cells.

BACKGROUND

The Field Effect Transistor

The transistor is a solid state semiconductor device which can be usedfor amplification, switching, voltage stabilization, signal modulationand many other functions. Generally, a transistor has three terminals,and a voltage applied to a specific one of the terminals controlscurrent flowing between the other two terminals. One type of transistoris known as the field effect transistor (FET).

The terminals of a field effect transistor (FET) are commonly namedsource (S), gate (G) and drain (D). In the FET, a small amount ofvoltage is applied to the gate (G) in order to control current flowingbetween the source (S) and the drain (D). In FETs the main currentappears in a narrow conducting channel formed near (usually primarilyunder) the gate. This channel connects electrons from the sourceterminal to the drain terminal. The channel conductivity can be alteredby varying the voltage applied to the gate terminal, enlarging orconstricting the channel and thereby controlling the current flowingbetween the source and the drain.

FIG. 1 illustrates a FET 100 comprising a p-type substrate (or a p-wellin the substrate), and two spaced-apart n-type diffusion areas—one ofwhich will serve as the “source”, the other of which will serve as the“drain” of the transistor. The space between the two diffusion areas isthe “channel”. A thin dielectric layer is disposed over the substrate inthe neighborhood of the channel, and a “gate” structure is disposed overthe dielectric layer atop the channel. (The dielectric under the gate isalso commonly referred to as “gate oxide” or “gate dielectric”.)Electrical connections (not shown) may be made to the source, the drain,and the gate. The substrate may be grounded, or it may be biased at adesired voltage depending on applications.

Generally, when there is no voltage on the gate, there is no electricalconduction (connection) between the source and the drain. As voltage (ofthe correct polarity) is applied to the gate, there is a “field effect”in the channel between the source and the drain, and current can flowbetween the source and the drain, and can be controlled by the voltageapplied to the gate. In this manner, a small signal (gate voltage) cancontrol a relatively large signal (current flow between the source andthe drain).

FET 100 is exemplary of a MOSFET (metal oxide semiconductor FET)transistor. With the specified “n” and “p” types shown above, an“n-channel MOSFET” can be formed. With opposite polarities (swapping “p”for “n” in the diffusions, and “n” for “p” in the substrate or well), ap-channel FET can be formed. In CMOS (complementary metal oxidesemiconductor), both n-channel and p-channel MOS transistors may beused, and are often paired with one another.

An integrated circuit (IC) device may comprise many millions of FETs ona single semiconductor “chip” (or “die”), measuring only a fewcentimeters on each side. Several IC chips may be formed simultaneously,on a single “wafer”, using conventional semiconductor fabricationprocesses including deposition, doping, photolithography, and etching.After all the chips are formed, they can be singulated from the wafer.

The Floating Gate Transistor

A floating gate transistor is generally a transistor structure, broadlybased on the FET, as described hereinabove. As illustrated in FIG. 2,the floating gate transistor 200 has a source and a drain, but ratherthan having only one gate, it has two gates which are called controlgate (CG) and floating gate (FG). It is this arrangement of control gateand floating gate which enables the floating gate transistor to functionas a memory cell, as described hereinbelow.

The floating gate is disposed over tunnel oxide (comparable to the gateoxide of the FET). The floating gate is a conductor, and the tunneloxide is an insulator (dielectric material). Another layer of oxide(interpoly oxide, also a dielectric material) separates the floatinggate from the control gate.

Since the floating gate is a conductor, and is surrounded by dielectricmaterial, it can store a charge. Electrons can move around freely withinthe conductive material of the floating gate (which comports with thebasic definition of a “conductor”).

Since the floating gate can store a charge, it can exert a field effecton the channel region between the source and the drain, in a mannersimilar to how a normal FET works, as described hereinabove. Mechanismsfor storing charges on the floating gate structure, as well as removingcharges from the floating gate, are described hereinbelow.

Generally, if a charge is stored on the floating gate, this represents abinary “1”. If no charge is stored on the floating gate, this representsa binary “0”. (These designations are arbitrary, and can be reversed sothat the charged state represents binary “0” and the discharged staterepresents binary “1”.) That represents the programming “half” of how afloating gate memory cell operates. The other half is how to determinewhether there is a charge stored on the floating gate—in other words, to“read” the memory cell. Generally, this is done by applying appropriatevoltages to the source, drain and gate terminals, and determining howconductive the channel is. Some modes of operation for a floating gatememory cell are described hereinbelow.

Normally, the floating gate non-volatile memory (NVM) cell has only asingle “charge-storing area”—namely, the conductive floating gate (FG)structure, and can therefore only store a single bit of information(binary “1” or binary “0”). More recently, using a technology referredto as “multi-level cell” (MLC), two or more bits can be stored in andread from the floating gate cell.

The NROM Memory Cell

Another type of memory cell, called a “nitride, read only memory” (NROM)cell, has a charge-storage structure which is different from that of thefloating gate memory cell and which permits charges to be stored (ortrapped) in two separate charge-storage areas. Generally, the twoseparate charge storage areas are located within a non-conductive layerdisposed between the gate and the underlying substrate, such as a layerof nitride formed in an oxide-nitride-oxide (ONO) stack underneath thegate. The non-conductive layer acts as a charge-trapping medium.Generally, electrical charges will stay where they are put in thecharge-trapping medium, rather than being free to move around as in theexample of the conductive floating gate of the floating gate memorycell. A first bit of binary information (binary “1” or binary “0”) canbe stored in a first portion (such as the left-hand side) of thecharge-trapping medium, and a second bit of binary information (binary“1” or binary “0”) can be stored in a second portion (such as theright-hand side) of the charge-trapping medium. An alternative viewpointis that different charge concentrations can be considered for each bitof storage. Using MLC technology, at least two bits can be stored in andread from each of the two portions (charge storage areas) of thecharge-trapping medium (for a total of 4 bits), similarly 3 bits or morethan 4 bits may be identified.

FIG. 3 illustrates a basic NROM memory cell 300, which may be viewed asan FET with an “ONO” structure inserted between the gate and thesubstrate. (One might say that the ONO structure is “substituted” forthe gate oxide of the FET.)

The ONO structure is a stack (or “sandwich”) of bottom (lower) oxide322, a charge-trapping material such as nitride 324, and a top (upper)oxide 326. The ONO structure may have an overall thickness ofapproximately 10-25 nm, such as 18 nm, as follows:

-   -   the bottom oxide layer 322 may be from 3 to 6 nm, for example 4        nm thick;    -   the middle nitride layer 324 may be from 3 to 8 nm, for example        4 nm thick; and    -   the top oxide layer 326 may be from 5 to 15 nm, for example 10        nm thick.

The NROM memory cell has two spaced apart diffusions 314 and 316 (whichcan function as source and drain, as discussed hereinbelow), and achannel region 320 defined in the substrate 312 between the twodiffusion regions 314 and 316, and a gate 328 disposed above the ONOstack 321.

In FIG. 3, the diffusions are labeled “N+”. This means that they areregions in the substrate that have been doped with an electron donormaterial, such as phosphorous or arsenic. These diffusions are typicallycreated in a larger region which is a p-type cell well (CW) doped withboron (or indium or both). This is the normal “polarity” for an NVM cellemploying electron injection (but which may also employ hole injection,such as for erase). With opposite polarity (boron or indium implants inan n-type cell well), the primary injection mechanism would be forholes, which is generally accepted to be not as effective as electroninjection. One skilled in the art will recognize that the conceptsdisclosed herein can be applied to opposite polarity devices.

The charge-trapping material 324 is non-conductive, and therefore,although electrical charges can be stored in the charge-trappingmaterial, they are not free to move around, they will generally staywhere they are stored. Nitride is a suitable charge-trapping material.Charge trapping materials other than nitride may also be suitable foruse as the charge-trapping medium. One such material is silicon dioxidewith buried polysilicon islands. A layer (324) of silicon dioxide withpolysilicon islands would be sandwiched between the two layers of oxide(322) and (326). Alternatively, the charge-trapping layer 324 may beconstructed by implanting an impurity, such as arsenic, into a layer ofsilicon dioxide deposited on top of the bottom oxide 322.

Memory cell 300 is generally capable of storing at least two bits ofdata—at least one bit(s) in a first storage area of the nitride layer324 represented by the dashed circle 323, and at least one bit(s) in asecond storage area of the nitride layer 324 represented by the dashedcircle 325. Thus, the NROM memory cell can be considered to comprise two“half cells”, each half cell capable of storing at least one bit(s). Itshould be understood that a half cell is not a physically separatestructure from another half cell in the same memory cell. The term “halfcell”, as it may be used herein, is used herein only to refer to the“left” or “right” bit storage area of the ONO stack 321 (nitride layer324). Storage areas 325, 323 may variously be referred to as “chargestorage areas”, “charge trapping areas”, and the like, throughout thisdocument. (The two charge storage areas may also be referred to as theright and left “bits”.)

Each of storage areas 325, 323 in the charge-trapping material 324 canexert a field effect on the channel region 320 between the source andthe drain, in a manner similar to how a normal FET works, as describedhereinabove (FIG. 2).

Generally, if a charge is stored in a given storage area of thecharge-trapping material, this represents a binary “1”, and if no chargeis stored in a given storage area of the charge-trapping material, thisrepresents a binary “0”. (Again, these designations are arbitrary, andcan be reversed so that the charged state represents binary “0” and thedischarged state represents binary “1”.) That represents the programming“half” of how an NROM memory cell operates. The other half is how todetermine whether there is a charge stored in a given storage area ofthe charge-trapping material—in other words, to “read” the memory cell.Generally, this is done by applying appropriate voltages to thediffusion regions (functioning as source and drain) and gate terminals,and determining how conductive the channel is.

Modes of Operation

Generally, the modes of operation for any NVM memory cell (eitherfloating gate or NROM) include “program”, “erase” and “read”. Modes ofoperation for NROM are now discussed.

Program generally involves injecting electrons into the charge storageareas of the NROM or other NVM cell, typically by a process known aschannel hot electron (CHE) injection. Exemplary voltages to program (byCHE injection of electrons) the right bit (right bit storage area) of anNROM cell may include:

-   -   the left BL (acting as source, Vs) is set to 0 volts    -   the right BL (acting as drain, Vd) is set to +5 volts    -   the gate (Vg) is set to +8-10 volts    -   the substrate (Vb) is set to 0 volts and the bit storage area        above the drain (right BL) becomes programmed To program the        left bit storage area, source and drain are reversed—the left        bitline serves as the drain and the right bitline serves as the        source.

Erase may involve injecting holes into the charge storage areas of theNROM cell, typically by a process known as hot hole injection (HHI).Generally, holes cancel out electrons (they are electrically opposite),on a one-to-one basis. Exemplary voltages to erase (by HHI injection ofholes) the right bit of an NROM cell may include:

-   -   the left BL (acting as source, Vs) is set to float    -   the right BL (acting as drain, Vd) is set to +5 volts    -   the gate (Vg) is set to −7 volts    -   the substrate (Vb) is set to 0 volts        and the bit storage area above the drain (right BL) becomes        erased. To erase the left bit storage area, source and drain are        reversed—the left bitline serves as the drain, and the right        bitline serves as the source.

Read may involve applying voltages to the terminals of the memory celland, based on subsequent current flow, ascertaining the thresholdvoltage of the charge storage area within the cell. Generally, to readthe right bit of the NROM cell, using “reverse read”,

-   -   the right BL (acting as source, Vs) is set to 0 volts    -   the left BL (acting as drain, Vd) is set to +2 volts    -   the gate (Vg) is set to +5 volts    -   the substrate (Vb) is set to 0 volts        and the bit storage area above the source (right BL) can be        read. To read the left bit storage area, source and drain are        reversed—the left bitline serves as the source, and the right        bitline serves as the drain.        “Reading” an NROM Cell

Reading an NROM memory cell may involve applying voltages to theterminals of the memory cell comparable to those used to read a floatinggate memory cell, but reading may be performed in a direction oppositeto that of programming Generally, rather than performing “symmetrical”programming and reading (as is the case with the floating gate memorycell, described hereinabove), the NROM memory cell is usually programmedand read “asymmetrically”, meaning that programming and reading occur inopposite directions. This is illustrated by the arrows in FIG. 3.Programming is performed in what is termed the forward direction andreading is performed in what is termed the opposite or reversedirection. For example, generally, to program the right storage area 323(in other words, to program the right “bit”), electrons flow from left(source) to right (drain). To read the right storage area 323 (in otherwords, to read the right “bit”), voltages are applied to cause electronsto flow from right to left, in the opposite or reverse direction. Forexample, generally, to program the left storage area 325 (in otherwords, to program the left “bit”), electrons flow from right (source) toleft (drain). To read the left storage area 325 (in other words, to readthe left “bit”), voltages are applied to cause electrons to flow fromleft to right, in the opposite or reverse direction. See, for example,U.S. Pat. No. 6,768,165.

“Read” is generally done by measuring the Vt of a cell (or half-cell),and associating the measured Vt with a program level (such as “0” or“1”). Although the Vts of the cells are measured on an individual basis,it is generally necessary to determine a distribution of Vts for manycells in order to associate the measured Vt of a given cell with aprogram level, with confidence. For example—if only one cell were to beread, and its threshold voltage were to be found to be at or very nearthe RV between two program levels, it may be difficult to say, withcertainty, at which of two program levels the single cell wasprogrammed, since its threshold voltage may have moved slightly upwardor slightly downward since it was programmed. This is a benefit ofreading bits one block at a time—to obtain a statistically meaningfulsample of Vt's across a number of cells.

Memory Array Architecture, Generally

Memory arrays are well known, and comprise a plurality (many, includingmany millions) of memory cells organized (including physically arranged)in rows (usually represented in drawings as going across the page,horizontally, from left-to-right) and columns (usually represented indrawings as going up and down the page, from top-to-bottom).

As discussed hereinabove, each memory cell comprises a first diffusion(functioning as source or drain), a second diffusion (functioning asdrain or source) and a gate, each of which has to receive voltage inorder for the cell to be operated, as discussed hereinabove. Generally,the first diffusions (usually designated “source”) of a plurality ofmemory cells are connected to a first bit line, which may be designated“BL(n)”, and second diffusions (usually designated “drain”) of theplurality of memory cells are connected to a second bit line which maybe designated “BL(n+1)”. Typically, the gates of a plurality of memorycells are connected to common word lines (WL).

The bitlines may be “buried bitline” diffusions in the substrate, andmay serve as the source/drain diffusions for the memory cells. Thewordlines may be polysilicon structures and may serve as the gateelements for the memory cells.

FIG. 4 illustrates an array of NROM memory cells (labeled “a” through“i”) connected to a number of word lines (WL) and bit lines (BL). Forexample, the memory cell “e” has its gate connected to WL(n), its source(left hand diffusion) is connected to BL(n), and its drain (right handdiffusion) is connected to BL(n+1). The nine memory cells illustrated inFIG. 4 are exemplary of many millions of memory cells that may beresident on a single chip.

Notice, for example that the gates of the memory cells “e” and “f” (tothe right of “e”) are both connected to the same word line WL(n). (Thegate of the memory cell “d” to the left of “e” is also connected to thesame word line WL(n).) Notice also that the right hand terminal(diffusion) of memory cell “e” is connected to the same bit line BL(n+1)as the left-hand terminal (diffusion) of the neighboring memory cell“f”. In this example, the memory cells “e” and “f” have two of theirthree terminals connected together.

The situation of neighboring memory cells sharing the sameconnection—the gates of neighboring memory cells being connected to thesame word line, the source (for example, right hand diffusion) of onecell being connected to the drain (for example left hand diffusion) ofthe neighboring cell—is even more dramatically evident in what is called“virtual ground architecture” wherein two neighboring cells actuallyshare the same diffusion. In virtual ground array architectures, thedrain of one memory cell may actually be the same diffusion which isacting as the source for its neighboring cell. Examples of virtualground array architecture may be found in U.S. Pat. Nos. 5,650,959;6,130,452; and 6,175,519, incorporated in their entirety by referenceherein.

The bitlines (BLs) mentioned above may extend in parallel with oneanother, through the array (or a portion thereof). A single buriedbitline (BB) may be a single diffusion shared by two adjacent celltransistors, and may be referred to herein as a diffusion bitline (DBL).The diffusion bitline (DBL) is a diffusion, functioning as a conductiveline, which is physically disposed (“buried”) within the surface of thesubstrate.

A cell transistor may be formed by an ONO stack, which is disposed onthe surface of the substrate, generally between adjacent bitlines (BB,dbl). The ONO stack includes a layer of silicon nitride (nitride), whichcan trap (store) electrons and holes. A gate electrode, which may bedoped (to be conductive) polysilicon, is disposed on the ONO stack, andmay be a portion of an elongate wordline extending through the array (ora portion thereof).

An inter-level dielectric (ILD) may be disposed on the substrate tosupport patterns of metallization for interconnecting cell transistors.A portion of the pattern may comprise metal bitlines (MBLs). Contacts tothe buried bitlines may be made by metal-filled plugs extending throughthe ILD to the buried bitlines, such as at intervals of every 16 cells.(It is generally not necessary to have one bitline contact per cell, anda contact area occupies area which otherwise could be used for celltransistors.)

In some of the memory array illustrations presented herein, thewordlines may extend horizontally through the array (or a portionthereof), and the bitlines may extend vertically through the array (or aportion thereof), intersecting the wordlines at right angles (90degrees) thereto. Or, in some of the illustrations, the memory array maybe rotated so that the wordlines extend vertically and the bitlinesextend horizontally.

Generally, the cell transistors described herein may be “dual bit”transistors, such as NROM, having two bit storage (charge-trapping)areas which may be referred to as the “left bit” and the “right bit”.

Drain-Side and Source-Side Sensing

Generally, for an NROM cell having two bit storage areas, one over thedrain (D) and another over the source (S), reading the bit over thesource (S) may be performed by setting the drain “read” voltage (Vdr) toapproximately +2 volts, such as 1.4 volts, and setting the sourcevoltage (Vs) to 0 volts (or ground). For example, with reference to FIG.3, to read the “right” bit 323, use the left diffusion 314 as drain (D)and the right diffusion 316 as source (S). To read the “left” bit 325,use the right diffusion 316 as drain (D) and the left diffusion 314 assource (S).

With Vdr on the drain (D), the program level (as represented by Vt)stored over the source (S) may be sensed. Generally, to sense the Vt,current is measured, such as the current flowing into the drain (D) orthe current coming out of the source (S). Either the drain (D) or thesource (S) may be used as the “sensing node”.

Parasitic Capacitances

An issue being addressed by the present disclosure is parasiticcapacitance, and the effects thereof.

Generally, in any electronic device, wherever there are two conductors(separated by an insulator), there are capacitances. In some cases,capacitors are advertently constructed, such as in DRAM cells forstoring charges, and are considered to be desirable. However, in manycases, the mere presence of conductive lines and elements can introduce“stray”, generally undesirable capacitances, referred to herein as“parasitic capacitances”. A reason that these parasitic capacitances aregenerally undesirable, in the context of reading memory cells, is thatthey can “steal” charge, by providing a “leakage” path. So, for example,when trying to read a memory cell programmed to a given voltage, themeasured current may appear sufficiently lower to result in a false(erroneous) reading. When current leaks, voltage drops, generallyaccording to the well known Ohm's law where E (voltage)=I (current)times R (resistance). More current leaking means more voltage dropping.

With so many conductive structures crammed together in very smallspaces, for example, when operating one metal bitline (MBL) to read thecontents of a memory cell, stray capacitance from adjacent MBLs (orLBLs) may cause voltage drops leading to false readings.

This issue is addressed, for example, in US Patent Publication No.2003/0202411, incorporated by reference in its entirety herein, whichdiscloses a system for control of pre-charge levels in a memory device.As noted therein, some terminals in an array of memory cells may becharged-up to prevent leakage currents which would otherwise adverselyinfluence reading the contents of memory cells.

Pipe Effect Problem Description

In a Virtual Ground Array (VGA), or any other topological architecturewhere there is an electrical connection between neighboring cells, thereis a possibility of current leaking to the neighboring cells, chargingthe parasitic capacitance associated with these cells (diffusion bitlines—DBLs) and other capacitances which may be connected to thesecells.

This parasitic current will affect the sensing current, and thus willcreate “cross-talk” between cells. (Cross-talk is when the value of onememory cell affects the sensed value of another, typically neighboring,cell. Cross-talk is a different issue than disturb or retention orendurance.)

A “pipe effect” is typically caused due to a difference between thedrain/source voltage level of the cell that is being read, to thepipe's, (neighbor cell's) voltage level. The pipe current can eitherincrease or reduce the sensed current, and thus change the apparentlogical value of the flash cell. This can be particularly troublesomewhen reading the contents of multi-level cells. The magnitude of thepipe problem depends on the neighboring flash cell's operating condition(erased or programmed), on the temperature, the word-line level, theparasitic capacitance and more. Thus, it cannot readily be compensatedby a simple shift in some parameter or another (such as gate voltage ordrain voltage, for example), but should be eliminated as much aspossible.

Commonly-owned patents disclose structure and operation of NROM andrelated ONO memory cells. Some examples may be found in commonly-ownedU.S. Pat. Nos. 5,768,192 and 6,011,725, 6,649,972 and 6,552,387.

Commonly-owned patents disclose architectural aspects of an NROM andrelated ONO array, (some of which have application to other types of NVMarray) such as segmentation of the array to handle disruption in itsoperation, and symmetric architecture and non-symmetric architecture forspecific products, as well as the use of NROM and other NVM array(s)related to a virtual ground array. Some examples may be found incommonly-owned U.S. Pat. Nos. 5,963,465, 6,285,574 and 6,633,496.

Commonly-owned patents also disclose additional aspects at thearchitecture level, including peripheral circuits that may be used tocontrol an NROM array, or the like. Some examples may be found incommonly-owned U.S. Pat. Nos. 6,233,180, and 6,448,750.

Commonly-owned patents also disclose several methods of operation ofNROM and similar arrays, such as algorithms related to programming,erasing, and/or reading such arrays. Some examples may be found incommonly-owned U.S. Pat. Nos. 6,215,148, 6,292,394 and 6,477,084.

Commonly-owned patents also disclose manufacturing processes, such asthe process of forming a thin nitride layer that traps hot electrons asthey are injected into the nitride layer. Some examples may be found incommonly-owned U.S. Pat. Nos. 5,966,603, 6,030,871, 6,133,095 and6,583,007.

Commonly-owned patents also disclose algorithms and methods of operationfor each segment or technological application, such as: fast programmingmethodologies in all flash memory segments, with particular focus on thedata flash segment, smart programming algorithms in the code flash andEEPROM segments, and a single device containing a combination of dataflash, code flash and/or EEPROM. Some examples may be found incommonly-owned U.S. Pat. Nos. 6,954,393 and 6,967,896.

Where applicable, descriptions involving NROM are intended specificallyto include related oxide-nitride technologies, including SONOS(Silicon-Oxide-Nitride-Oxide-Silicon), MNOS(Metal-Nitride-Oxide-Silicon), MONOS (Metal-Oxide-Nitride-Oxide-Silicon)and the like used for NVM devices. Further description of NVM andrelated technologies may be found at “Non Volatile Memory Technology”,Vol. 1 & 2 (2005), Vol. 3 (2006) and Vol. 4 (2007), published by SaifunSemiconductor; “Microchip Fabrication”, by Peter Van Zant, 5^(th)Edition 2004; “Application-Specific Integrated Circuits” by Michael JohnSebastian Smith, 1997; “Semiconductor and Electronic Devices”, by AdirBar-Lev, 2^(nd) Edition, 1999; “Digital Integrated Circuits” by Jan M.Rabaey, Anantha Chandrakasan and Borivoje Nikolic, 2^(nd) Edition, 2002and materials presented at and through http://siliconnexus.com, “DesignConsiderations in Scaled SONOS Nonvolatile Memory Devices” found at:http://klabs.org/richcontent/MemoryContent/nvmtsymp/nvmts_(—)2000/presentations/buwhitesonos_lehigh_univ.pdf, “SONOS Nonvolatile Semiconductor Memories forSpace and Military Applications” found at:http://klabs.org/richcontent/MemoryContent/nvmtsymp/nvmts_(—)2000/papers/adams_d.pdf,“Philips Research-Technologies-Embedded Nonvolatile Memories” found at:http://www.research.philips.com/technologies/ics/nvmemoriesindex.html,and “Semiconductor Memory: Non-Volatile Memory (NVM)” found at:http://www.ece.nus.edu.sg/stfpage/elezhucx/myweb/NVM.pdf,

all of which are incorporated by reference herein in their entirety.

GLOSSARY

Unless otherwise noted, or as may be evident from the context of theirusage, any terms, abbreviations, acronyms or scientific symbols andnotations used herein are to be given their ordinary meaning in thetechnical discipline to which the disclosure most nearly pertains. Thefollowing terms, abbreviations and acronyms may be used throughout thedescriptions presented herein and should generally be given thefollowing meaning unless contradicted or elaborated upon by otherdescriptions set forth herein. Some of the terms set forth below may beregistered trademarks (®).

When glossary terms (such as abbreviations) are used in the description,no distinction should be made between the use of capital (uppercase) andlowercase letters. For example “ABC”, “abc” and “Abc”, or any othercombination of upper and lower case letters with these 3 letters in thesame order, should be considered to have the same meaning as oneanother, unless indicated or explicitly stated to be otherwise. The samecommonality generally applies to glossary terms (such as abbreviations)which include subscripts, which may appear with or without subscripts,such as “X_(yz)” and “Xyz”. Additionally, plurals of glossary terms mayor may not include an apostrophe before the final “s”—for example, ABCsor ABC's.

-   array memory cells may optionally be organized in an array of rows    and columns, and may be connected to selected bit lines and word    lines in the array. The array may be organized into various logical    sections containing pluralities of memory cells, such as blocks,    pages and sectors. Some of these sections may be physically isolated    and operated independently from one another.-   bit The word “bit” is a shortening of the words “binary digit.” A    bit refers to a digit in the binary numeral system (base 2). A given    bit is either a binary “1” or “0”. For example, the number 1001011    is 7 bits long. The unit is sometimes abbreviated to “b”. Terms for    large quantities of bits can be formed using the standard range of    prefixes, such as kilobit (Kbit), megabit (Mbit) and gigabit (Gbit).    A typical unit of 8 bits is called a Byte, and the basic unit for    128 Bytes to 16K Bytes is treated as a “page”. That is the    “mathematical” definition of “bit”. In some cases, the actual    (physical) left and right charge storage areas of an NROM cell are    conveniently referred to as the left “bit” and the right “bit”, even    though they may store more than one binary bit (with MLC, each    storage area can store at least two binary bits). The intended    meaning of “bit” (mathematical or physical) should be apparent from    the context in which it is used.-   BL short for bit line. The bit line is a conductor connected to the    drain (or source) of a memory cell transistor.-   byte A byte is commonly used as a unit of storage measurement in    computers, regardless of the type of data being stored. It is also    one of the basic integral data types in many programming languages.    A byte is a contiguous sequence of a fixed number of binary bits. In    recent years, the use of a byte to mean 8 bits is nearly ubiquitous.    The unit is sometimes abbreviated to “B”. Terms for large quantities    of Bytes can be formed using the standard range of prefixes, for    example, kilobyte (KB), megabyte (MB) and gigabyte (GB).-   erase a method to erase data on a large set of bits in the array,    such as by applying a voltage scheme that inject holes or remove    electrons in the bit set. This method causes all bits to reach a low    Vt level.-   FET short for field effect transistor. The FET is a transistor that    relies on an electric field to control the shape and hence the    conductivity of a “channel” in a semiconductor material. FETs are    sometimes used as voltage-controlled resistors. The terminals of    FETs are called gate (G), drain (D) and source (S).-   FG short for floating gate. The floating-gate transistor is a kind    of transistor that is commonly used for non-volatile storage such as    flash, EPROM and EEPROM memory. Floating-gate transistors are almost    always floating-gate MOSFETs. Floating-gate MOSFETs are useful    because of their ability to store an electrical charge for extended    periods of time even without a connection to a power supply.-   Flash memory Flash memory is a form of non-volatile memory (EEPROM)    that can be electrically erased and reprogrammed Flash memory    architecture allows multiple memory locations to be erased or    written in one programming operation. Two common types of flash    memory are NOR and NAND flash. NOR and NAND flash get their names    from the structure of the interconnections between memory cells. In    NOR flash, cells are connected in parallel to the bit lines,    allowing cells to be read and programmed individually. The parallel    connection of cells resembles the parallel connection of transistors    in a CMOS NOR gate. In NAND flash, cells are connected in series,    resembling a NAND gate, and preventing cells from being read and    programmed individually: the cells connected in series must be read    in series.    -   Some systems will use a combination of NOR and NAND memories,        where a smaller NOR memory is used as software ROM, and a larger        NAND memory is partitioned with a file system for use as a        random access storage area. NAND is best suited to flash devices        requiring high capacity data storage. This type of flash        architecture combines higher storage space with faster erase,        write, and read capabilities over the execute in place advantage        of the NOR architecture. See NAND, NOR.-   half cell “half cell” (or “half-cell”) is a term which is sometimes    used to refer to the two distinct charge storage areas (left and    right bits) of an NROM memory cell.-   ILD short for inter-level (or inter-layer) dielectric. Generally,    ILD is a relatively thick layer of oxide deposited on completed    underlying structures (such as FETs), which will support a layer(s)    of metal lines interconnecting the various underlying structures.    Holes may be etched through the ILD and filled with metal to make    contact with elements (such as source, drain, gate) of the    underlying structures.-   MLC short for multi-level cell. In the context of a floating gate    (FG) memory cell, MLC means that at least two bits of information    can be stored in the memory cell. In the context of an NROM memory    cell, MLC means that at least four bits of information can be stored    in the memory cell—at least two bits in each of the two charge    storage areas.-   NAND NAND flash architecture memories are accessed much like block    devices such as hard disks or memory cards. The pages are typically    512 or 2,048 or 4,096 bytes in size. Associated with each page are    usually a few bytes (typically 12-16 bytes) that are used for    storage of an error detection (ED) and correction checksum. The    pages are typically arranged in blocks, such as 32 pages of 512    bytes, 64 pages of 2,048 bytes, or 64 pages of 4,096 bytes. With    NAND architecture, programming may be performed on a page basis, but    erasure can only be performed on a block basis.    -   Most NAND devices are shipped from the factory with some bad        blocks, which are typically identified and marked according to a        specified bad block marking strategy. The first physical block        (block 0) is always guaranteed to be readable and free from        errors. Hence, all vital pointers for partitioning and bad block        management for the device are located inside this block        (typically a pointer to the bad block tables). If the device is        used for booting a system, this block may contain the master        boot record.-   nitride commonly used to refer to silicon nitride (chemical formula    Si3N4). A dielectric material commonly used in integrated circuit    manufacturing. Forms an excellent mask (barrier) against oxidation    of silicon (Si). Nitride is commonly used as a hard mask or, in the    case of an NVM memory cell having an ONO layer, as a charge-trapping    material.-   NOR Reading from NOR flash is similar to reading from random-access    memory, provided the address and data bus are mapped correctly.    Because of this, most microprocessors can use NOR flash memory as    execute in place (XIP) memory, meaning that programs stored in NOR    flash can be executed directly without the need to copy them into    RAM. NOR flash chips lack intrinsic bad block management, so when a    flash block is worn out, the software or device driver controlling    the device must handle this, or the device will cease to work    reliably.-   NROM short for nitride(d) read only memory. Generally, a FET-type    device having a charge trapping medium such as a nitride layer for    storing charges (electrons and holes) in two discrete areas, near    the source and drain diffusions, respectively.-   NVM short for non-volatile memory. NVM is computer memory that can    retain the stored information even when not powered. Examples of    non-volatile memory include read-only memory, flash memory, most    types of magnetic computer storage devices (for example hard disks,    floppy disk drives, and magnetic tape), optical disc drives, and    early computer storage methods such as paper tape and punch cards.    Non-volatile memory is typically used for the task of secondary    storage, or long-term persistent storage. The most widely used form    of primary storage today is a volatile form of random access memory    (RAM), meaning that when the computer is shut down, anything    contained in RAM is lost. Unfortunately most forms of non-volatile    memory have limitations which make it unsuitable for use as primary    storage. Typically non-volatile memory either costs more or performs    worse than volatile random access memory. (By analogy, the simplest    form of an NVM memory cell is a simple light switch. Indeed, such a    switch can be set to one of two (binary) positions, and “memorize”    that position.) NVM includes floating gate (FG) devices and NROM    devices, as well a devices using optical, magnetic and phase change    properties of materials.-   ONO short for oxide-nitride-oxide. ONO is used as a charge storage    insulator consisting of a sandwich of thermally insulating oxide,    and charge-trapping nitride.-   oxide commonly used to refer to silicon dioxide (SiO2). Also known    as silica. SiO2 is the most common insulator in semiconductor device    technology, particularly in silicon MOS/CMOS where it is used as a    gate dielectric (gate oxide); high quality films may be obtained by    thermal oxidation of silicon. Thermal SiO2 forms a smooth,    low-defect interface with Si, and can be also readily deposited by    CVD.-   poly short for polycrystalline silicon (Si). Heavily doped poly Si    is commonly used as a gate contact in silicon MOS and CMOS devices;-   program a method to program a bit in an array, by applying a voltage    scheme that injects electrons. This method causes an increase in the    Vt of the bit that is being programmed. Alternatively, with “high Vt    erase”, programming may be a lowering of the Vt of the memory cell.    See erase and read. Program may sometimes, erroneously be referred    to as “write”. See write.-   read a method to read the digital data stored in the array. The read    operation is usually performed in “blocks” of several cells. See    erase and program.-   ROM short for read-only memory.-   SA short for sense amplifier (or “sense amp”). A sense amplifier is    generally used in conjunction with memory arrays to amplify a    voltage coming off of a bit line. There are many versions of sense    amplifiers used in memory chips.-   sector a part of the array, usually larger than a page, which    usually contains a few pages. A minimum erase might include a    sector. For example:    -   Erase Sector (ES): Group of cells that are erased by single        erase command    -   Physical Sector (PS): Group of ES connected by single grid of        Word Lines (WL) and Bit Lines (BL), sharing same set of drivers.-   SLC short for single level cell. In the context of a floating gate    (FG) memory cell, SLC means that one bit of information can be    stored in the memory cell. In the context of an NROM memory cell,    SLC means that at least two bits of information can be stored in the    memory cell.-   slice a portion of a memory array, such as a group of bitlines, that    are connected to one sense amplifier (sense amp, “SA”)-   virtual ground array a topological architecture of memory cells    where there is an electrical connection between neighboring cells.-   V short for voltage. Different voltages may be applied to different    parts of a transistor or memory cell to control its operation-   word line or wordline, (WL). A conductor normally connected to the    gate of a memory cell transistor. The wordline may serve as the gate    electrode of several memory cells.-   write a combined method usually involving first erasing a large set    of bits, then programming new data into the bit set; the erase step    is not required but it is customary. See erase and program.

In addition to the above, some abbreviations or terminology that may beused herein, or in a provisional application (if any) from which thisnon-provisional application claims priority, may include:

-   add short for address-   bot short for bottom-   BS short for byte select-   cmi representative of a voltage indicative of the current coming out    of a cell being read-   CS short for chunk select. Also referred to as column select-   DBL short for diffusion bit line. May also refer to local bitline    (LBL)-   FC short for flash cell-   GBL short for global bit line. May also refer to metal bitline    (MBL).-   Icell short for current going through a cell-   LBL short for local bit line. May also refer to diffusion bitline    (DBL).-   MBL short for metal bit line. May also refer to global bitline    (GBL).-   sl, sel short for select-   Vdr short for drain voltage-   Vdr_ave short for average drain voltage-   Vtn short for NMOS threshold voltage

BRIEF DESCRIPTION (SUMMARY)

According to some embodiments of the present invention, there isprovided a method of reading one or more non-volatile memory (“NVM”)cells in an NVM array, including pre-charging or discharging one or morebitlines which are not directly connected to said one or more NVM cellsbeing read. Reading of the one or more NVM cells may be performed in anorder which keeps a sensing node of the cells being read facing abitline which has been pre-charged or discharged. Pre-charging ordischarging of bitlines may include pre-charging or discharging severalglobal bitlines and local bitlines. The pre-charged or discharged globalbit lines may be selected according to a pre-defined table per each celladdress. Selection of global bitlines may be done according to whetherthese global bitlines will interfere with sensing during the next readcycle.

According to further embodiments of the present invention, there may beprovided a method of reading a plurality of non-volatile memory (NVM)cells which are arranged in a row of memory cells and which areselectively connected to a first bitline and a second bitline comprisingreading a first memory cell in the row, skipping the second adjacentcell, and reading a third memory cell in the row, keeping charged orkeeping discharged bitlines of the memory cell which was previouslyread. A sequence for reading additional memory cells in the same row mayskip adjacent cells to cells that were read and may keep charged ordischarged bitlines of those memory cells that were read. The sequencemay be repeated, reading one or more memory cells that were previouslyskipped by skipping one or more memory cells that were previously read.

According to some embodiments of the present invention, there may beprovided a method of reading a plurality of non-volatile memory (NVM)cells which are arranged in a row of memory cells and which areselectively connected to a first bitline and a second bitline, themethod including using a read order such that a charged or dischargedstate remains on the drain-side or on the source-side of the cells. Themethod may include keeping charged or keeping discharged drain-side orsource-side diffusion bitlines of memory cells which were previouslyread.

According to further embodiments of the present invention, there may beprovided a method of reading a plurality of non-volatile memory (NVM)cells which are arranged in a row of memory cells, by using one of thecell's two diffusion bitlines as a sensing node and performing read inan order which keeps a sensing node facing a bitline which waspre-charged or discharged. The pre-charged or discharged bitlines may beselected so as to mitigate pipe effect during a next read cycle.

According to some embodiments of the present invention, there may beprovided a non-volatile memory (“NVM”) device including NVM cells in anNVM array and control logic adapted to cause a circuit to pre-charge ordischarge one or more bitlines not directly connected with the one ormore NVM cells. The control logic may be adapted to cause the reading ofthe one or more NVM cells to occur in an order which keeps a sensingnode of the cells being read facing a bitline which has been pre-chargedor discharged. Pre-charging or discharging comprises pre-charging ordischarging several global bitlines and local bitlines. Pre-charged ordischarged global bit lines may be selected according to a pre-definedtable per each address. Selection of the global bitlines may be doneaccording to whether these global bitlines will interfere with sensingduring the next read cycle.

According to further embodiments of the present invention, anon-volatile memory (“NVM”) device may include a plurality ofnon-volatile memory (NVM) cells which are arranged in a row of memorycells and which are selectively connected to a first bitline and asecond bitline comprising. The device may include control logic adaptedto apply a cell read order that causes a charged or discharged state toremain on the drain-side or source-side of the cells. The control logicmay be adapted to cause switching circuitry to keep charged or keepdischarged drain-side or source-side diffusion bitlines of memory cellswhich were previously read.

An object of the disclosure is to provide an improved sensing techniquefor non-volatile memory cells, such as (but not limited to) NROM cells.

Generally, memory cells arranged in a NAND architecture will bediscussed. Generally, a number of memory cells are connected to a commonwordline (WL), and a given memory cell may be formed between twodiffusion bitlines (DBLs) which are connected by select (SEL)transistors to respective top and bottom metal bitlines (MBLs).

Memory cells may be referred to as flash cells (FCs). Diffusion bitlines(DBLs) may be referred to as local bitlines (LBLs). Metal bitlines(MBLs) may be referred to as global bitlines (GBL).

During sensing (reading the contents of a memory cell), there may be a“pipe effect” created by conductive elements (such as adjacent orneighboring FCs, DBLs, MBLs) which are connected to the memory cell (FC)being sensed, acting as capacitors and draining current. This cansignificantly reduce the accuracy of a sensing scheme, and may beparticularly troublesome with multi-level cell (MLC) sensing schemeswhere the different program levels tend to be very closely spaced.

According to the disclosure, generally, a dynamic pre-charge sequence(method) may significantly reduce pipe current and/or neighbor current.The method may also include a dedicated read order of the flash cells,which will result in keeping the sensing node facing the section of thepipe which was pre-charged.

The method may include pre-charging (or discharging) several metal bitlines (MBLs) and diffusion bitlines (DBLs) substantially to the bitlineread voltage (V_(BLR), or Vdr). The pre-charged MBLs may be selectedaccording to a pre-defined table per each address. The selection of theMBLs may also be done according to whether these MBLs will interferewith the pipe during the next read cycle. Alternatively, the pre-chargecan be to a higher level than required, and then discharged to the“required” voltage.

Generally, by pre-charging (or pre-discharging) the pipe to a controlledlevel, the pipe (neighbor) current is significantly reduced and, thus,the sensing current is almost equal to the flash cell current, which iswhat a good sensing scheme tries to achieve.

The techniques disclosed herein can be applied for both drain-side andsource-side sensing schemes. In the main hereinafter, drain-side sensing(or simply “drain sensing”) schemes are discussed.

The pipe effect can significantly degrade flash performance. A method tosignificantly reduce pipe current and/or neighbor current using apre-charge sequence is disclosed. A dedicated read order keeps thesensing node facing the section of the pipe which was pre-charged. Thetechnique involves pre-charging several global bitlines (such as metalbitlines, or MBLs) and local bitlines (such as diffusion bitlines, orDBLs). The pre-charged global bitlines may be selected according to apre-defined table per each address. The selection of the global bitlinesmay be done according to whether these global bitlines will interferewith the pipe during the next read cycle.

By using the techniques disclosed herein, the pipe is pre-conditioned,and performance degradation is minimized (lessened), resulting in betteroverall performance of the memory array.

According to an embodiment of the disclosure, a method to reduce pipecurrent when reading an array of flash cells comprises: using a dynamicpre-charge sequence. Reading may be performed in an order which keeps asensing node of the cells being read facing a portion of the pipe whichhas been pre-charged.

According to an aspect of the disclosure, pre-charging may comprisepre-charging several global bitlines and local bitlines. The pre-chargedglobal bit lines may be selected according to a pre-defined table pereach address. The selection of the global bitlines may be done accordingto whether these global bitlines will interfere with the pipe during thenext read cycle.

According to an embodiment of the disclosure, a method of reading aplurality of non-volatile memory (NVM) cells which are arranged in a rowof memory cells and which are selectively connected to a first metalbitline (MBL-0) and a second metal bitline (MBL-1) comprises: reading afirst memory cell, skipping the next, reading the next, skipping thenext, and repeating this pattern, starting at one end of the row andterminating at an opposite end of the row. Read-side diffusion bitlinesof memory cells which were previously read may be kept charged.

According to an embodiment of the disclosure, a method of reading aplurality of non-volatile memory (NVM) cells which are arranged in a rowof memory cells and which are selectively connected to a first metalbitline (MBL-0) and a second metal bitline (MBL-1) comprises: organizinga read order so that a charge remains on the read-side of the cells. Thememory cells may be selectively connected to the metal bitlines byselect transistors (Mn) and respective diffusion bitlines (DBL-n).Read-side diffusion bitlines of memory cells which were previously readmay be kept charged.

According to an embodiment of the disclosure, a method of reading aplurality of non-volatile memory (NVM) cells which are arranged in a rowof memory cells, using one of the cell's two diffusions as a sensingnode, comprises: performing read in an order which will result inkeeping the sensing node facing the section of the pipe which waspre-charged. Global bitlines and local bitlines may be pre-charged. Thepre-charged global bitlines may be selected according to a pre-definedtable per each address. The pre-charged global bitlines may be selectedso as to minimize pipe effect during a next read cycle.

According to an embodiment of the disclosure, a method of sequentiallyreading a slice of flash cells, one side of the slice constituting apipe, comprises: starting read at one end of the slice, and changing theaddress, such that the pipe will not be discharged. A sensing node sidemay be kept facing the same direction so that the metal bitlinesassociated with the cells being read do not change state. Whenever asignificant amount of capacitance is added to the pipe side, apre-charge pulse may be asserted in order to minimize the pipe effect.If a first required read address is located somewhere other than at theend of the slice, the pipe may first be pre-charged then, after the pipeis pre-charged, the read address may be selected.

Advantages of the techniques disclosed herein may include:

-   -   reduced pipe effect    -   more accurate sensing

The techniques disclosed herein may be applicable to most NVM devicesincluding, but not limited to, charge-trapping devices such as NROM(sometimes referred to as Nitride Read Only Memory), SONOS(Semiconductor Oxide Nitride Oxide Semiconductor;Silicon-Oxide-Nitride-Oxide-Silicon), SANDS (Silicon-AluminumOxide-Nitride-Oxide-Silicon), MANOS (Metal-AluminumOxide-Nitride-Oxide-Silicon), and TANOS (Tantalum-AluminumOxide-Nitride-Oxide-Silicon), and also to Floating Gate (FG) devices.

BRIEF DESCRIPTION OF THE DRAWING(S)

Reference will be made in detail to embodiments of the disclosure,examples of which may be illustrated in the accompanying drawing figures(FIGS). The figures are intended to be illustrative, not limiting.Although the disclosure is generally described in the context of theseembodiments, it should be understood that it is not intended to limitthe disclosure to these particular embodiments.

Certain elements in selected ones of the figures may be illustratednot-to-scale, for illustrative clarity. The cross-sectional views, ifany, presented herein may be in the form of “slices”, or “near-sighted”cross-sectional views, omitting certain background lines which wouldotherwise be visible in a true cross-sectional view, for illustrativeclarity. In some cases, hidden lines may be drawn as dashed lines (thisis conventional), but in other cases they may be drawn as solid lines.

If shading or cross-hatching is used, it is intended to be of use indistinguishing one element from another (such as a cross-hatched elementfrom a neighboring un-shaded element). It should be understood that itis not intended to limit the disclosure due to shading or cross-hatchingin the drawing figures.

Elements of the figures may (or may not) be numbered as follows. Themost significant digits (hundreds) of the reference number correspond tothe figure number. For example, elements of FIG. 1 are typicallynumbered in the range of 100-199, and elements of FIG. 2 are typicallynumbered in the range of 200-299. Similar elements throughout thefigures may be referred to by similar reference numerals. For example,the element 199 in FIG. 1 may be similar (and possibly identical) to theelement 299 in FIG. 2. Throughout the figures, each of a plurality ofelements 199 may be referred to individually as 199 a, 199 b, 199 c,etc. Such relationships, if any, between similar elements in the same ordifferent figures, will become apparent throughout the specification,including, if applicable, in the claims and abstract.

Throughout the descriptions set forth in this disclosure, lowercasenumbers or letters may be used, instead of subscripts. For example Vgcould be written V_(g). Generally, lowercase is preferred to maintainuniform font size.) Regarding the use of subscripts (in the drawings, aswell as throughout the text of this document), sometimes a character(letter or numeral) is written as a subscript-smaller, and lower thanthe character (typically a letter) preceding it, such as “V_(s)” (sourcevoltage) or “H₂O” (water). For consistency of font size, such acronymsmay be written in regular font, without subscripting, using uppercaseand lowercase—for example “Vs” and “H2O”.

Conventional electronic components may be labeled with conventionalschematic-style references comprising a letter (such as A, C, Q, R)indicating the type of electronic component (such as amplifier,capacitor, transistor, resistor, respectively) followed by a numberindicating the iteration of that element (such as “1” meaning a first oftypically several of a given type of electronic component). Componentssuch as resistors and capacitors typically have two terminals, which maybe referred to herein as “ends”. In some instances, “signals” arereferred to, and reference numerals may point to lines that carry saidsignals. In the schematic diagrams, the various electronic componentsare connected to one another, as shown. Usually, lines in a schematicdiagram which cross over one another and where there is a dot at theintersection of the two lines are connected with one another, else (ifthere is no dot at the intersection) they are typically not connectedwith one another.

FIG. 1 is a stylized cross-sectional view of a field effect transistor(FET), according to the prior art. To the left of the figure is aschematic symbol for the FET.

FIG. 2 is a stylized cross-sectional view of a floating gate memorycell, according to the prior art. To the left of the figure is aschematic symbol for the floating gate memory cell.

FIG. 3 is a stylized cross-sectional view of a two bit NROM memory cellof the prior art. To the left of the figure is a schematic symbol forthe NROM memory cell.

FIG. 4 is a diagram of a memory cell array with NROM memory cells,according to the prior art.

FIGS. 5-13 are schematic illustrations of a row (or slice) of memorycells, illustrating an embodiment of a method of reading the memorycells, according to the disclosure.

FIGS. 14-17 are schematic illustrations of a row (or slice) of memorycells, illustrating an embodiment of a method of reading the memorycells, according to the disclosure.

DETAILED DESCRIPTION

In the following description, various aspects of techniques for readingof NVM cells/arrays will be described. For the purpose of explanation,specific configurations and details are set forth in order to provide athorough understanding of the techniques. However, it will also beapparent to one skilled in the art that the techniques may be practicedwithout specific details being presented herein. Furthermore, well-knownfeatures may be omitted or simplified in order not to obscure thedescription(s) of the techniques.

Although various features of the disclosure may be described in thecontext of a single embodiment, the features may also be providedseparately or in any suitable combination. Conversely, although thedisclosure may be described herein in the context of separateembodiments for clarity, the disclosure may also be implemented in asingle embodiment. Furthermore, it should be understood that thedisclosure can be carried out or practiced in various ways, and that thedisclosure can be implemented in embodiments other than the exemplaryones described herein below. The descriptions, examples, methods andmaterials presented in this specification, as well as in the claims,should not be construed as limiting, but rather as illustrative.

The disclosure is directed to a sensing technique for reading NVM(non-volatile memory), particularly FCs (flash cells). The sensingmechanism includes a pre-charge phase, integrated with a dedicated readorder to reduce pipe effect (neighbor current) and provide more accuratesensing, particularly of multi-level cell flash memory. The techniquemay also be useful for single-level flash memory cells.

Generally, before a new read cycle starts, or whenever MBL drain ischanged, a pre-charge operation (phase) may be done. Alternatively,first all of the MBLs are in a charged condition, then discharged. Thesetwo (charging from zero up to a voltage, or discharging down to thevoltage) should be considered to be interchangeable, for purposes ofthis disclosure and, in either case, the pre-charge phase may beperformed before there is a change in MBL voltage which otherwise wouldlead to parasitic capacitance.

As explained in greater detail hereinbelow, for each address thatrequires the pre-charge phase, the relevant metal bitlines (MBLs) may bepre-charged to substantially the VBLR (bitline read voltage) level forthe memory cell which will be read. The selection of which MBLs topre-charge per address is based on the slice architecture and may becontrolled by software (SW). (Generally, a “slice” may be a plurality ofmemory cells associated with one sense amplifier (SA).) Selecttransistors are turned on, charging the associated diffusion bitlines(DBLs) through the MBLs. Generally, by pre-charging appropriate MBLs,the effects of parasitic capacitance (pipe effect) can be minimized,thereby increasing read accuracy. DBLs can be selected/controlled basedon slice architecture by software as well as MBLs.

After the pre-charge phase, the MBLs and DBLs may be kept floating, andthe charge stored on them blocks the pipe current during the active readcycle.

A dedicated read order may be chosen to minimize the necessity for thepre-charge phase and minimize the pipe effect. As explained in greaterdetail hereinbelow, by observing the dedicated read order, the need topre-charge before reading may be avoided, thereby eliminating somesteps. For example, by reading every other cell, from left-to-right(such as FC-1, FC-3, FC-5, FC-7 . . . ), a pre-charge may only berequired for the initial read of FC-1, as described in greater detailhereinbelow.

An Exemplary Solution

An exemplary solution to the pipe effect problem, in the case ofdrain-side sensing, may comprise the following steps, set forth first inoutline form:

1. Pre-Charging the Pipe Prior to the Read Cycle:

-   -   a. Pipe address/configuration is selected    -   b. Pre-charge pulse is asserted        -   Pre-charging most of the DBLs and all or some of the MBLs            which would be affecting the pipe on the drain side of the            FC being read, during the coming read cycle    -   c. Address changes to read address

2. Read Cycle

-   -   a. Pre-charge stopped, and the drain side is supplied by a        controlled voltage (through the drain driver)        -   Icell stabilization phase    -   b. after allowing for stabilization, sensing starts

For source-side sensing, the same steps would be followed, but on thesource side, rather than the drain side. Either node (source or drain)may be used for sensing. In the main, hereinafter, drain-side sensing isdiscussed.

Two main cases of operation will be discussed:

-   -   a. sequential read cycles        -   Start read at one end of an “isolated” slice, and change the            address such that the pipe will not be discharged. (An            “isolated” slice is a slice that has no neighbors/pipe at            the drain side.)            -   This may be achieved by keeping the drain side facing                the same direction, so that the MBLs don't change state                (for example don't switch their functionality between                drain or source)        -   Whenever a significant amount of capacitance is added to the            pipe side (for example: MBL change), a pre-charge pulse is            asserted in order to minimize the pipe effect.    -   b. one read cycle        -   If the first required read address is located somewhere in            the middle (other than at the end) of the slice, then the            pipe is being pre-charged first, and only afterwards the            read address is selected as described above            An Exemplary Sequential Read Cycle

An exemplary sequential read cycle (or sequence) will now be discussed.Generally, this sequence starts from an isolated end of a slice.

The exemplary read cycle sequence is presented in the following Table(“Table 1. Sequential Read Cycle”), and is illustrated with respect toFIGS. 5-13. Table 1 summarizes an example of a decoding configurationduring pre-charge and read cycle phases. The example is for a sequentialread starting from the left isolated end of the slice->address 0 toaddress 7.

Table 1 describes the state of the select transistors and MBLs and DBLsduring the read cycle, and defines the state of the select and MBLsduring the read cycle pre-pulse address phase used for pre-charge (ordischarge).

Generally, each row of Table 1, from top down, represents a step in thesequential read cycle. Each of the rows will now be described,generally. What is happening at each step, more particularly withrespect to the eight rows labeled logic add 0 through 7, is illustratedin (and described in greater detail with respect to) FIGS. 5-13.

As mentioned above, with Vdr on the drain (D), the program level (Vt)stored over the source (S) may be sensed. Generally, to sense the Vt,current is measured, such as the current flowing into the drain (D) orthe current coming out of the source (S). Either the drain (D) or thesource (S) may be used as the “sensing node”.

Table 1 accounts for a number of FCs #0-#14 (compare FCs a-o; FIG. 5)connected to a common wordline, and illustrates that:

-   -   logical address “0” is associated with FC-0 (“a” in FIG. 5),    -   logical address “1” is associated with FC-2 (“c” in FIG. 5),    -   logical address “2” is associated with FC-4 (“e” in FIG. 5),    -   and so forth.

Generally, as the logical address increments by one (+1), the physicaladdress increments by two (+2), skipping every other FC, to maintain asequence of drain-side sensing.

For source-side sensing, a similar scheme of skipping every other FC canbe implemented, based on the teachings presented herein, such as formemory cells FC-1 (“b” in FIG. 5), FC-3 (“d” in FIG. 5), FC-5 (“f” inFIG. 5), and so forth.

TABLE 1 Sequential Read Cycle Add logic Cell SL DBL MBL add letter FC# tb D S D S — all all discharged 0 a 0 0 0 0 1 0 1 1 c 2 0, 1 1 2 3 0 1 2e 4 0, 1, 2 2 4 5 0 1 3 g 6 0, 1, 2, 3 3 6 7 0 1 4 i 8 0, 1, 2, 3, 4 4 89 0 1 5 k 10 0, 1, 2, 3, 4, 5 5 10 11 0 1 0, 1, 2, 3, 4, 5 1, 2, 3,Pre-charge 0, 1 4, 5 6 m 12 0 0 12 13 2 3 7 o 14 0, 1 1 14 15 2 3

Table 1 will now be discussed, briefly, row-by-row, cross-referencingselected ones of FIGS. 5-13.

The cycle starts with discharge of all the slice. All of the MBLs aredischarged. The read will start from the isolated end of the slice, or“wall”, to the left of dbl<0>.

The first address (Logical Address 0, FC#0, memory cell “a”) is suchthat the dbl drain (0) is facing the isolated end of the slice.Therefore, there is no pipe, and no pre-charge phase is necessary. SeeFIG. 5.

The second address (Logical Address 1, FC#2, memory cell “c”) uses MBL-0for drain read voltage (Vdr) and MBL-1 for source (grounded). The drainDBL is (2), since select bot (0) is “off” there is no problem usingMBL-0 as source. See FIG. 6.

The same applies for Logical Addresses 2-5 (FC#4, FC#6, FC#8, FC#10)where the number of select top which are turned on is increased to helpcharging the DBLs during the read cycle itself, making the pre-chargephase optional. See FIGS. 7-10.

Then, a pre-charge step is performed, prior to reading Logical Address6. See FIG. 11.

For reading Logical Address 6 (FC#12, cell “m” in FIG. 5), MBL-2 is usedas drain, and MBL-3 is used as source instead of MBL-1. Note that MBL-1(which was grounded) would be connected to the pipe through the selectbot 0, which is used during the coming read cycle. Therefore MBL-1 needsa pre-charge. A pre-charge pulse is sent to MBL-0 and MBL-1 and, usingthe top select transistors, the DBLs to the left of the next drain(DBL-12) are also charged. See FIG. 12.

The above setting maintains the pipe facing the drain (DBL-12) charged,and thus may significantly reduce the pipe effect.

Neglecting to do the above pre-charge flow would have allowed a largepipe effect to manifest itself, specifically because MBL-1 would beconnected to the pipe, and because its capacitance is large and it is atground level, and it would have discharged the pipe to some extent whichwould have created a parasitic pipe current affecting the read ofLogical Address 6.

For reading Logical Address 7, (FC#14, cell “o” in FIG. 5), MBL-2 isused for the drain and MBL-3 is used for the source.

For reading Logical Address 8, (FC#16, cell “q” in FIG. 5), MBL-2 isused for the drain and MBL-3 is used for the source.

This scheme may continue, in the manner set forth above, until the rightisolated end of the slice is reached. Flash cells which were “skipped”(FC#1, FC#3, FC#5 . . . ), in order to keep the drains facing onedirection (left), may be read in a similar manner, as describedhereinbelow. The alternative of using source-side sensing is alsodiscussed below.

FIGS. 5-13 are schematic illustrations of a portion (which may be a“slice”) of a memory array, illustrating an embodiment of the techniquesdisclosed herein. Generally, as used herein, a “slice” is a number ofmemory cells that are associated with one sense amp (SA).

FIG. 5 illustrates a portion of a memory array, generally comprising aplurality (24 shown) of memory cells “a” through “l ”, and “m” through“x” having their gates connected to a common wordline WL (compare WLn,FIG. 4) The memory cells may be NROM cells, such as illustrated anddescribed with respect to FIG. 3. Additional rows of memory cells (notshown) may be connected to other wordlines in the array (such as WLn−1or WLn+1, FIG. 4). The memory cells are suitably “flash” memory cells,which may be abbreviated “FC”, for “flash cell”.

The wordline(s) may generally extend in the “X” direction, horizontallyacross the array, and the bitlines (described hereinbelow) may generallyextend in the “Y” direction, vertically across the array.

For illustrative purposes, the memory cells may be arranged in thefollowing manner. Memory cell b is to the right of memory cell a, memorycell c is to the right of memory cell b, memory cell d is to the rightof memory cell c, memory cell e is to the right of memory cell d, memorycell f is to the right of memory cell e, memory cell g is to the rightof memory cell f, memory cell h is to the right of memory cell g,continuing (alphabetically, in this manner) to memory cell x is to theright of memory cell w, as illustrated. It should be noted that:

-   -   memory cells a, c, e, g, i, k comprise a group of (six) memory        cells,    -   memory cells b, d, f, h, j, l comprise a group of (six) memory        cells,    -   memory cells m, o, q, s, u, w comprise a group of (six) memory        cells,    -   memory cells n, p, r, t, v, x comprise a group of (six) memory        cells,

The purpose of these groupings of memory cells will become apparent inlight of the following description. The number of memory cells in agroup is at least 2.

Each memory cell has a right side diffusion and a left side diffusion,and a memory cell is generally symmetrical, left-to-right. Depending onhow the memory cell is being operated, the right side diffusion mayoperate as a source (S) or drain (D), and the left side diffusion mayserve as the drain (D) or source (S). For example, in the case of anNROM cell, to read the right bit of the cell,

-   -   the right diffusion, acting as a source, may be set to 0 volts        (ground)    -   the left diffusion, acting as a drain, may be set to        approximately +2 volts (such as +1.4 v)    -   the gate (Vg) may be set to approximately +5 volts    -   the substrate (Vb) may be set to 0 volts (or ground)

and to read the left bit of the cell,

-   -   the left diffusion, acting as a source, may be set to 0 volts        (ground)    -   the right diffusion, acting as a drain, may be set to        approximately +2 volts    -   the gate (Vg) may be set to approximately +5 volts    -   the substrate (Vb) may be set to 0 volts (or ground)

The memory cells may be physically arranged so that they sharediffusions. For example, the right side diffusion of the memory cell “a”may also be the left side diffusion of the memory cell “b”, the rightside diffusion of the memory cell “b” may also be the left sidediffusion of the memory cell “c”, and the right side diffusion of thememory cell “c” may also be the left side diffusion of the memory cell“d”, and so forth (as shown), and the right side diffusion of the memorycell “w” may also be the left side diffusion of the memory cell “x”, asillustrated.

A plurality of diffusion bitlines (DBLs) are illustrated. The diffusionbit lines may comprise the source/drain diffusions of the memory cells,as well as other rows of memory cells. Twenty four diffusion bitlines(DBLs) are shown, and are numbered from “0” to “23”. In the drawing, agiven diffusion bitline DBL<n> may be referred to in the text as DBL-n(using a dash “-” instead of angle brackets “< >”), where “n” is anumber between 0 and 23. The diffusion bitlines may be arranged, asfollows:

-   -   a diffusion bit line (DBL-0) extends to the left side diffusion        of the memory cell “a”;    -   a diffusion bit line (DBL-1) extends to the right side diffusion        of the memory cell “a”, which may also be the left side        diffusion of the memory cell “b”;    -   a diffusion bit line (DBL-2) extends to the left side diffusion        of the memory cell “c”, which may also be the right side        diffusion of the memory cell “b”;    -   and so forth (as shown), until    -   a diffusion bitline (DBL-23) extends to the right side diffusion        of the memory cell “w”, which may also be the left side        diffusion of the memory cell “x”, as illustrated.

A number of metal bitlines (MBLs) are illustrated. Four metal bitlines(MBLs) are shown, and are numbered from “0” to “3”. In the drawing, agiven metal bitline mbl<n> may be referred to in the text as MBL-n(using a dash “-” instead of angle brackets “< >”), where “n” is anumber between 0 and 3. Each diffusion bitline (DBL) may be connected,through a select transistor (Mn) to a metal bitline (MBL) as follows.

-   -   diffusion bitlines DBL-0, DBL-2, DBL-4, DBL-6, DBL-8 and DBL-10,        which comprise the left diffusions of memory cells a, c, e, g,        i, and k are connected through corresponding select transistors        M0, M2, M4, M6, M8 and M10 to metal bitline (MBL-0)    -   diffusion bitlines DBL-1, DBL-3, DBL-5, DBL-7, DBL-9 and DBL-11,        which comprise the left diffusions of memory cells b, d, f, h,        j, and l are connected through corresponding select transistors        M1, M3, M5, M7, M9 and M11 to metal bitline (MBL-1)    -   diffusion bitlines DBL-12, DBL-14, DBL-16, DBL-18, DBL-20 and        DBL-22, which comprise the left diffusions of memory cells m, o,        q, s, u, and w are connected through corresponding select        transistors M12, M14, M16, M18, M20 and M22 to metal bitline        (MBL-3)    -   diffusion bitlines DBL-13, DBL-15, DBL-17, DBL-19, DBL-21 and        DBL-23, which comprise the left diffusions of memory cells n, p,        r, t, v, and x are connected through corresponding select        transistors M13, M15, M17, M19, M21 and M23 to metal bitline        (MBL-3)

In this example,

-   -   six select transistors (M0, M2, M4, M6, M8, M10) are associated        with six diffusion bitlines (DBL-0, DBL-2, DBL-4, DBL-6, DBL-8,        DBL-10), with eleven memory cells (a, b, c, d, e, f, g, h, i, j,        k), and with one metal bitline MBL-0. (The reason that there are        only eleven, rather than twelve memory cells associated with        MBL-0 is because this example is showing the left-hand side, or        “isolated end” of the slice.)    -   six select transistors (M1, M3, M5, M7, M9, M11) are associated        with six diffusion bitlines (DBL-1, DBL-3, DBL-5, DBL-7, DBL-9,        DBL-11), with twelve memory cells (a, b, c, d, e, f, g, h, i, j,        k, l), and with one metal bitline MBL-1.    -   six select transistors (M12, M14, M16, M18, M20, M22) are        associated with six diffusion bitlines (DBL-12, DBL-14, DBL-16,        DBL-18, DBL-20, DBL-22), with twelve memory cells (l, m, n, o,        p, q, r, s, t, u, v, w), and with one metal bitline MBL-3.    -   six select transistors (M13, M15, M17, M19, M21, M23) are        associated with six diffusion bitlines (DBL-13, DBL-15, DBL-17,        DBL-19, DBL-21, DBL-23), with twelve memory cells (m, n, o, p,        q, r, s, t, u, v, w, x), and with one metal bitline MBL-4.        (Here, in contrast with MBL-0, the slice is shown continuing        onward beyond cell x, so there are twelve memory cells        associated with MBL-3.)

Generally, when a select transistor (M) is turned “on” (or is “up”), aconnection between the corresponding diffusion bitline (DBL) and thecorresponding metal bitline (MBL) is made. Else, if the selecttransistor is not “on”, the connection between the diffusion bitline(DBL) and the metal bitline (MBL) is not made (open, no connection, or“down”). The select transistors may be turned “on” under the control of(in response to) signals provided on two select buses (SBs). Two selectbuses are shown—a “top” select bus sl_top<5:0> which may be referred toin the text as SB-top, and a “bottom” select bus sl_bot<5:0> which maybe referred to in the text as SB-bot. For example,

-   -   the select transistors M0, M2, M4, M6, M8, and M10 may be        selectively controlled (switched “on”) by a signal from a select        bus (SB-top). Note that these select transistors M0, M2, M4, M6,        M8, and M10 may all be associated with the left side diffusions        of alternate (every other) memory cell a, c, e, g . . . .    -   the select transistors M1, M3, M5, M7, M9, and M11 may be        selectively controlled (switched “on” by a signal from a select        bus (SB-bot). Note that these select transistors M1, M3, M5, M7,        M9, and M11 may all be associated with the left side diffusion        of alternate (every other) memory cells b, d, f, h . . . .    -   the select transistors M12, M14, M16, M18, M20, and M22 may be        selectively controlled (switched “on”) by a signal from a select        bus (SB-top). Note that these select transistors M12, M14, M16,        M18, M20, and M22 may all be associated with the left side        diffusions of alternate (every other) memory cell m, o, q, s . .        . .    -   the select transistors M13, M15, M17, M19, M21, and M23 may be        selectively controlled (switched “on”) by a signal from a select        bus (SB-bot). Note that these select transistors M13, M15, M17,        M19, M21, and M23 may all be associated with the left side        diffusion of alternate (every other) memory cells n, p, r, t . .        . .

The select buses (SBs) control which select transistors (Ms) are turnedon, and when a select transistor (M) is turned on, a voltage which is onthe metal bitline (MBL) to which the select transistor (M) is connectedwill be imposed upon the diffusion bitline (DBL) to which the transistor(M) is connected. Generally, in the description that follows, thevoltage on the Metal Bitlines (MBLs)—which is supplied via externaldrivers, decoders and/or multiplexers (not shown)—will be either zero(or ground), approximately +2.0 volts (such as +1.4 volts), or floating(not connected).

Generally, a given select bus (SB-top or SB-bot) controls selecttransistors in groups of six (in this example), corresponding to thelogical groupings of memory cells discussed above. (The terms “SB-top”and “sl_top” may be used interchangeably, and the terms “SB bot” and“sl_bot” may be used interchangeably, in the description that follows.)For example:

-   -   the select transistors M0 and M12 may be turned on in response        to a signal sl_top<0> (or simply “<0>”) from SB-top,    -   the select transistors M2 and M14 may be turned on in response        to a signal sl_top<1> (or simply “<1>”) from SB-top,    -   the select transistors M4 and M16 may be turned on in response        to a signal sl_top<2> (or simply “<2>”) from SB-top,    -   the select transistors M6 and M18 may be turned on in response        to a signal sl_top<3> (or simply “<3>”) from SB-top,    -   the select transistors M8 and M20 may be turned on in response        to a signal sl_top<4> (or simply “<4>”) from SB-top,    -   the select transistors M10 and M22 may be turned on in response        to a signal sl_top<5> (or simply “<5>”) from SB-top,    -   the select transistors M1 and M13 may be turned on in response        to a signal sl_bot<0> (or simply “<0>”) from SB-bot,    -   the select transistors M3 and M15 may be turned on in response        to a signal sl_bot<1> (or simply “<1>”) from SB-bot,    -   the select transistors M5 and M17 may be turned on in response        to a signal sl_bot<2> (or simply “<2>”) from SB-bot,    -   the select transistors M7 and M19 may be turned on in response        to a signal sl_bot<3> (or simply “<3>”) from SB-bot,    -   the select transistors M9 and M21 may be turned on in response        to a signal sl_bot<4> (or simply “<4>”) from SB-bot,    -   the select transistors M11 and M23 may be turned on in response        to a signal sl_bot<5> (or simply “<5>”) from SB-bot,

Generally, in a given cycle of six signals, the signals <1> follow thesignals <0>, the signals <2> follow the signals <1>, the signals <3>follow the signals <2>, the signals <4> follow the signals <3>, and thesignals <5> follow the signals <4>. For example, the overall cycle maybe approximately 1.5 μs (microseconds), and each individual signal maylast for approximately one-sixth of a cycle, or approximately 250 ns(nanoseconds).

It should be understood that the choice of six cells in the readsequences is arbitrary, the number should be at least two, and can be ashigh as desired. However, for larger numbers of cells, the delay imposedin reading may be undesirable.

Generally, the signals <0> from both SB-top and SB-bot may occursimultaneously with one another, the signals <1> from both SB-top andSB-bot may occur simultaneously with one another, the signals <2> fromboth SB-top and SB-bot may occur simultaneously with one another, thesignals <3> from both SB-top and SB-bot may occur simultaneously withone another, the signals <4> from both SB-top and SB-bot may occursimultaneously with one another, and the signals <5> from both SB-topand SB-bot may occur simultaneously with one another. This is but one ofmany optional combinations. Compare, for example, Table 1. Also, theremay be cases where bot<1> and top<1> may not be active together. Theinvention is not limited to the combinations which are shown anddescribed.

Note that FIG. 5 illustrates a plurality of memory cells comprising aleft hand portion of a slice, which may comprise additional memory cellsextending to the right. In the drawing, some lines are followed by threedots ( . . . ) to the right, indicating that they may continue onward.For example,

-   -   the top select bus SL-top <5:0> may continue, to the right    -   there may be more metal bitlines, such as MBL-4 and MBL-5 (not        shown), each associated with select transistors, diffusion        bitlines and memory cells    -   the wordline may continue, to the right    -   there may be additional memory cells, to the right of cell “x”    -   the bottom select bus SL-bot <5:0> may continue, to the right

Note also that in FIG. 5, there are circles around M0 and M1, and arrows(with legends “D” and “S”) pointing to the left and right diffusions ofmemory cell “a”. These are discussed below.

FIGS. 5-13, Illustrating the Sequential Read Cycle

In the following description of a technique for pre-charging metalbitlines in conjunction with reading the contents of memory cells,generally the left side diffusions of the memory cells being read areoperated as drain (D), such as with a voltage of approximately +2V (suchas +1.4 volts), and the right side diffusions of the memory cells beingread are operated as source (S), such as with a voltage of 0V (orground), such as for reading the contents of the right bit storage areasof the memory cells. It should be noted that the right side associatedwith source and left side associated with drain applies only to readingthe cells from left to right, but at some point the read sequence willcontinue to read the “second” bits, and then the sides will flip.

Various signals and states associated with the technique illustrated anddescribed with respect to FIGS. 5-13 are presented and/or tabulated inTable 1 (above) and in Table 2 (below).

Various voltages may selectively be applied to the various metalbitlines (MBL-0, MBL-1, MBL-2, MBL-3), as follows:

-   -   0 volts, or ground, which means that the MBL is grounded.    -   Vdr, which means that the appropriate read voltage (such as        approximately +2 volts) is applied to the MBL, either to        pre-charge it to Vdr, or in the process of reading a given        memory cell. Also referred to as bitline read voltage (V_(BLR)).    -   “float”, which means that no voltage is applied to the MBL. A        given MBL may be permitted to float “with charge” (after having        previously been charged, before it has an opportunity to        discharge), or “without charge” (which is generally a “don't        care” situation where the MBL may be retaining a previous charge        or previously has been discharged).

TABLE 2 Voltage Levels on Metal Bitlines Address FIG. MBL-0 MBL-1 MBL-2MBL-3 Read FIG. 5 Vdr 0 v (gnd) 0 v, or 0 v, or addr 0 float 0 v float 0v read FIG. 6 Vdr 0 v (gnd) 0 v, or 0 v, or addr 1 float 0 v float 0 vread FIG. 7 Vdr 0 v (gnd) 0 v, or 0 v, or addr 2 float 0 v float 0 vread FIG. 8 Vdr 0 v (gnd) 0 v, or 0 v, or addr 3 float 0 v float 0 vread FIG. 9 Vdr 0 v (gnd) 0 v, or 0 v, or addr 4 float 0 v float 0 vread FIG. 10 Vdr 0 v (gnd) 0 v, or 0 v, or addr 5 float 0 v float 0 vPre FIG. 11 Vdr Vdr float Float charge read FIG. 12 Float Float Vdr 0 v(gnd) addr 6 at Vdr at Vdr read FIG. 13 Float Float Vdr 0 v (gnd) addr 7at Vdr at Vdr

FIG. 5 illustrates a first step (Sequential read: Address 0) in thetechnique. For example,

-   -   A signal <0> from sl_top turns on select transistor M0. This is        indicated by the circle around M0. MBL-0 is set to 2 volts        (Vdr).    -   The select transistor M12 is also turned on. MBL-2 is set to 0        volts (gnd), or is left floating.    -   A signal <0> from sl_bot turns on select transistor M1. This is        indicated by the circle around M1. MBL-1 is set to 0 volts        (gnd).    -   The select transistor M13 is also turned on. MBL-3 is set to 0        volts (gnd), or is left floating.

The “convention” of drawing a circle around the select transistors (M)that are turned “on” will be adhered to in the following figures.

With these two transistors M0 and M1 turned on, memory cell “a” can beread, using “drain-side” or source side sensing. This represents readingof “Address 0”. (Refer to Table 1, logical address “0” corresponds toFC# “0”, cell letter “a”). The left diffusion of memory cell “a”,functioning as drain (D), is connected to metal bitline MBL-0, and theright diffusion of memory cell “a”, functioning as source (S), isconnected to metal bitline MBL-1. In an NROM cell, this would be“reverse read” of the right storage area (adjacent the source(S)) of thememory cell “b”.

Although the two transistors M12 and M13 are turned on, the associatedMBLs are left floating, so no reading of memory cell “m” is takingplace.

FIG. 6 illustrates a next step (Sequential read: Address 1) in thetechnique. For example,

-   -   A signal <1> from sl_top turns on select transistor M2. This is        indicated by the circle around M2. MBL-0 is set to 2 volts        (Vdr).    -   The select transistor M0 may be left turned on, by signal <0>        from sl_top.    -   The select transistor M12 may be turned on by the signal <0>,        and the select transistor M14 may also be turned on by the        signal <1>. MBL-2 remains floating.    -   A signal <1> from sl_bot turn on select transistor M3. This is        indicated by the circles around M3. MBL-1 is set to 0 volts        (gnd).    -   The select transistor M15 may be turned on. MBL-3 remains        floating.

With the two transistors M2 and M3 turned on, memory cell “c” can beread, using drain-side or source side sensing. This represents readingof “Address 1”. (Refer to Table 1, logical address “1” corresponds toFC# “2”, cell letter “c”). The left diffusion of memory cell “c”,functioning as drain (D), is connected to metal bitline MBL-0, and theright diffusion of memory cell “c”, functioning as source (S), isconnected to metal bitline MBL-1.

Notice in FIG. 6 that according to Table 1) sl_top<1> is on and so issl_top<0> (which was also on in the previous read cycle), and sl_bot<1>.This will charge the pipe which is faced by the drain side, that is,dbl's 0 and 2 will directly charged by MBL0 through sl_top<1> andsl_top<2> (dbl 2 is the active drain side) and dbl 1 will be chargedthrough FC ‘b’ from dbl's 0 and 2, while dbl 3 is the source sidethrough MBL1 and sl_bot<1>.

Although the two transistors M14 and M15 are turned on (M12 is alsoturned on), the associated MBLs are left floating, so no reading ofmemory cell “0” is taking place.

Note that in the previous (first)) step (FIG. 5), memory cell “a”(logical address 0, FC-0) was read, and in this (next) step (FIG. 6)memory cell “c” (logical address 1, FC-2, in Table 1) is read. This isindicative of the read sequence (or “read order”) that will be followed,involving reading every other cell in the row of the array, skippingover one cell in each subsequent step.

Note also in FIG. 6 that both bitlines DBL-0 and DBL-2 are charged (+2v) during the read cycle, and that M1 is turned off (note, it is notcircled). Thus, the pipe to the left of the active DBL (DBL<2>) ischarged, thereby reducing the pipe current and increasing the sensingaccuracy.

FIG. 7 illustrates a next step (Sequential read: Address 2) in thetechnique. For example,

-   -   A signal <2> from sl_; top turns on select transistor M4. This        is indicated by the circle around M4. MBL-0 is set to Vdr The        select transistors M0 and M2 may also be left on (and are        circled)    -   The select transistor M16 is also turned on. MBL-2 remains        floating. The select transistors M12 and M14 may also be left on        (and are circles).    -   A signal <2> from sl_bot turn on select transistor M5 (circled),        and MBL-1 is set to ground.    -   The select transistor M17 may also be turned on. MBL-3 remains        floating.

With the two transistors M4 and M5 turned on, memory cell “e” can beread, using drain-side or source side sensing. This represents readingof “Address 2”. (Refer to Table 1, logical address “2” corresponds tofloating cell “4”). The left diffusion of memory cell “e”, functioningas drain (D), is connected to metal bitline MBL-0, and the rightdiffusion of memory cell “e”, functioning as source (S), is connected tometal bitline MBL-1.

Although the two transistors M16 and M17 are turned on, the associatedMBLs are left floating, so no reading of memory cell “q” is takingplace. (The transistors M12 and M14 may also be turned on.)

Note that in the previous step (FIG. 6), memory cell “c” (logicaladdress 1, FC-2) was read, and in this (next) step (FIG. 7) memory cell“e” (logical address 2, FC-4, in Table 1) is read. This is indicative ofthe read sequence (or “read order”) that will be followed, involvingreading every other cell in the row of the array, skipping over one cellin each subsequent step.

Note also in FIG. 7 that the bitlines DBL-0, DBL-2 and DBL-4 are charged(+2 v) during the read cycle, and that M1 and M3 are turned off (notcircled). Thus, the pipe to the left of the active DBL (DBL<4>) ischarged, thereby reducing the pipe current and increasing the sensingaccuracy.

FIG. 8 illustrates a next step (Sequential read: Address 3) in thetechnique. For example,

-   -   A signal <3> from sl_top turns on select transistor M6. This is        indicated by the circle around M6. MBL-0 is set to Vdr. The        select transistors M0, M2 and M4 may be left turned on (by        signals <0>, <1> and <2> from sl_top).    -   The select transistor M18 is also turned on. MBL-2 remains        floating. The select transistors M12, M14 and M16 may be left        turned on.    -   A signal <3> from sl_bot turn on select transistor M7. MBL-1 is        grounded. Select transistor M19 may similarly be turned on.        MBL-3 remains floating.

With the two transistors M6 and M7 turned on, memory cell “g” can beread, using drain-side or source side sensing. This represents readingof “Address 3”. (Refer to Table 1, logical address “3” corresponds tofloating cell “6”). The left diffusion of memory cell “g”, functioningas drain (D), is connected to metal bitline MBL-0, and the rightdiffusion of memory cell “g”, functioning as source (S), is connected tometal bitline MBL-1.

Although the two transistors M18 and M19 are turned on, the associatedMBLs are left floating, so no reading of memory cell “t” is takingplace. (M12, M14 and M16 are also turned on.)

Note that in the previous step (FIG. 7), memory cell “e” (logicaladdress 2, FC-4) was read, and in this (next) step (FIG. 8) memory cell“g” (logical address 3, FC-6, in Table 1) is read. This is indicative ofthe read sequence (or “read order”) that will be followed, involvingreading every other cell in the row of the array, skipping over one cellin each subsequent step.

Note also in FIG. 8 that the bitlines DBL-0, DBL-2, DBL-4 and DBL-6 arecharged (+2 v) during the read cycle, and that M1, M3 and M5 are turnedoff (not circled). Thus, the pipe to the left of the active DBL (DBL<6>)is charged, thereby reducing the pipe current and increasing the sensingaccuracy.

FIG. 9 illustrates a next step (Sequential read: Address 4) in thetechnique. For example,

-   -   A signal <4> from sl_top turns on select transistor M8. This is        indicated by the circle around M8. MBL-0 is set to Vdr. Select        transistors M0, M2, M4, M6 may remain “on”.    -   The select transistor M20 is also turned on. MBL-2 remains        floating.    -   A signal <4> from sl_bot turn on select transistor M9. MBL-1 is        set to gnd. The select transistor M21 may be turned on. MBL-3        remains floating.

With the two transistors M8 and M9 turned on, memory cell “i” can beread, using drain-side or source side sensing. This represents readingof “Address 4”. (Refer to Table 1, logical address “4” corresponds tofloating cell “8”). The left diffusion of memory cell “i”, functioningas drain (D), is connected to metal bitline MBL-0, and the rightdiffusion of memory cell “i”, functioning as source (S), is connected tometal bitline MBL-1.

Although the transistors M20 and M21 are turned on, the associated MBLsare left floating, so no reading of memory cell “u” is taking place.

Note that in the previous step (FIG. 8), memory cell “g” (logicaladdress 3, FC-6) was read, and in this (next) step (FIG. 9) memory cell“i” (logical address 4, FC-8, in Table 1) is read. This is indicative ofthe read sequence (or “read order”) that will be followed, involvingreading every other cell in the row of the array, skipping over one cellin each subsequent step.

Note also in FIG. 9 that the bitlines DBL-0, DBL-2, DBL-4, DBL-6 andDBL-8 are charged (+2 v) during the read cycle, and that M1, M3, M5 andM7 are turned off (not circled). Thus, the pipe to the left of theactive DBL (DBL<8>) is charged, thereby reducing the pipe current andincreasing the sensing accuracy.

FIG. 10 illustrates a next step (Sequential read: Address 5) in thetechnique. For example,

-   -   A signal <5> from sl_top turns on select transistor M10. This is        indicated by the circle around M10. MBL-0 is set to Vdr. Select        transistors M0, M2, M4, M6, M8 may remain “on”.    -   The select transistor M22 is also turned on. MBL-2 remains        floating.    -   A signal <5> from sl_bot turn on select transistor M11. MBL-1 is        set to gnd. The select transistor M23 may be turned on. MBL-3        remains floating.

With the two transistors M10 and M11 turned on, memory cell “k” can beread, using drain-side sensing. This represents reading of “Address 5”.(Refer to Table 1, logical address “5” corresponds to FC#10). The leftdiffusion of memory cell “k”, functioning as drain (D), is connected tometal bitline MBL-0, and the right diffusion of memory cell “k”,functioning as source (S), is connected to metal bitline MBL-1.

Although the transistors M22 and M23 are turned on, the associated MBLsare left floating, so no reading of memory cell “w” is taking place.

FIGS. 5-10 have illustrated a sequence of reading memory cells “a”, “c”,“e”, “g”, “i” and “k”, which are arranged in a row of memory cells andwhich are selectively connected to metal bitline 0 (MBL-0) and metalbitline 1 (MBL-1), for reading the contents (stored Vt) of the memorycells.

A first cell “a” is read, the next cell “b” is not read (is skipped),the next cell “c” is read, the next cell “d” is not read, and so forth,as described above. What is evident is that the read order is organizedso that the charge remains on the left, on the drain-side (fordrain-side sensing, in other words, on the read-side or sensing node) ofthe cell being read. This minimizes the possibility of there being anystray capacitance on the drain-side of the cell being read, therebyenhancing read performance.

There has thus been described various aspects of the present disclosure,including, but not limited to:

-   -   maintaining the metal bitlines (MBLs) for a group of cells being        read at given voltages (such as MBL-0=0 volts and MBL-1=+2        volts),    -   reading in an order which skips every other cell, and    -   keeping open (on) the select transistors of neighboring cells        which were previously read so as to keep their drain (read-side)        bitlines (DBLs) charged.

FIG. 11 illustrates a next step (pre-charge phase) in the technique.Prior to reading cell “m” (logic add 6, cell letter “m”, FC#12, seeTable 1), a pre-charge step is performed. For example,

-   -   A signal <0> and a signal <1> from sl_top turns on select        transistors M0 and M2, as indicated by the circles around M0 and        M2, and MBL-0 is maintained at Vdr.    -   Select transistors M12 and M14 may also be turned on, but MBL-2        is maintained at float.    -   Select signals <1>, <2>, <3> and <4> from sl_bot turn on select        transistors M3, M5, M7 and M9 (and may also turn on select        transistors M15, M17, M19 and M21), as indicated by the circles        around M3, M5, M7 and M9, and MBL-1. MBL-1 is charged to Vdr.    -   Select transistors M15, M17, M19 and M21 may also be turned on,        but MBL-3 remains floating.

No reading is occurring in this step. Rather, the metal bitlines MBL-0and MBL-1 are being pre-charged for reading the cell “m”, between (M12and M13) that will be read in the next phase. This is indicated by thedashed-line ( - - - ) circle around cell “m”.

A pre-charge pulse is supplied to MBL-1, as is indicated by the arrowpointing up to MBL-1, generally under select transistor M7. Also, apre-charge pulse is supplied to MBL-0, as indicated by the arrowpointing down to MBL-0, generally above select transistor M6. Thepre-charge pulse may be at substantially the read voltage (Vdr), such as+2 volts.

A pre-charge current flows from MBL-1, through the associated at leastone select transistors (M3, M5, M7 and M9) that are turned on, and fromMBL-0 through the associated at least one select transistors (M0, M2)that are turned on. These current paths are indicated by:

-   -   the arrows pointing up along DBL-3, DBL-5, DBL-7 and DBL-9,    -   the arrows pointing down along DBL-0 and DBL-2, and    -   the arrows pointing along the wordline WL between cells “a” and        “j”.

The main reason for the pre-charge step is to prepare MBL<1> to the nextread cycle, because it was at ground level before, and in the next readcycle (as will be shown) it will be connected to the pipe through selecttransistor M1. Failing to pre-charge may cause the pipe to be dischargedby MBL<1> in the next read cycle due to MBL<1>'s relatively largecapacitance. In addition, turning select transistors M3, M5, M7, M9, M0and M2 charge the pipe left to the next active drain (DBL<12>) preparingit to the next read cycle.

FIG. 12 illustrates a next step (Sequential read: Address 6) in thetechnique. For example,

-   -   A signal <0> from sl_top turns on select transistor M12. This is        indicated by the circle around M12. MBL-2 is set to 0 volts        (ground).    -   The select transistor M0 is also turned on. MBL-0 is left        floating (at Vdr, from pre-charge phase).    -   A signal <0> from sl_bot turns on select transistor M13. MBL-3        is set to Vdr.    -   The select transistor M1 is also turned on. MBL-1 is left        floating (at Vdr, from pre-charge phase).

With the two transistors M12 and M13 turned on, memory cell “m” can beread, using drain-side or source-side sensing. This represents readingof “Address 5”. (Refer to Table 1, logical address “6” corresponds tofloating cell “12”). The left diffusion of memory cell “m”, functioningas drain (D), is connected to metal bitline MBL-3, and the rightdiffusion of memory cell “m”, functioning as source (S), is connected tometal bitline MBL-2.

M0 and M1 are also on, but MBL1 and MBL0 are float, so there is no readfrom cell “a”.

FIG. 13 illustrates a first step (Sequential read: Address 7) in thetechnique. For example,

-   -   Signal <0> and <1> from sl_top turns on select transistors M12        and M14. MBL-2 is set to Vdr.    -   The select transistors M0 and M2 are also turned on. MBL-0 is        left floating (at Vdr, from pre-charge phase).    -   A signal <1> from sl_bot turns on select transistor M15. MBL-3        is set to 2 volts.    -   The select transistor M3 is also turned on. MBL-1 is left        floating (at Vdr, from pre-charge phase).

With these two transistors M14 and M15 turned on, memory cell “o” can beread, using drain-side or source-side sensing. This represents readingof “Address 7”. (Refer to Table 1, logical address “7” corresponds tofloating cell “14”). The left diffusion of memory cell “o”, functioningas drain (D), is connected to metal bitline MBL-2, and the rightdiffusion of memory cell “o”, functioning as source (S), is connected tometal bitline MBL-3.

M2 and M3 are also on, but MBL1 and MBL0 are float, so there is no readfrom cell “c”.

The remaining cells “q”, “s”, “u” and “w” . . . can be read followingthe scheme (procedures, strategy) outlined above.

Reading the Other Bits, and the Other Cells

As described above, with respect to FIGS. 5-13, the right bits ofalternate memory cells “a”, “c”, “d”, “g”, “i”, “k” and “m” (forexample) may be performed. In each case, the left diffusion (see 325,FIG. 3) is used as drain (D) by being set to Vdr (approximately 2volts), with the right diffusion (see 323, FIG. 3) being used as source(S) and set to 0 volts (ground). Generally, MBL-1 provides Vdr, andMBL-0 provides ground.

In order to read the other bit of the same memory cells, the voltages onMBL-0 and MBL-1 can simply be reversed, so that MBL-0 provides Vdr, andMBL-1 provides ground.

In order to read the other memory cells “b”, “d”, “f”, “h”, “j”, “l ”and “n” (for example), the following may be done. For example, to readthe right bit of memory cell “b”, set MBL-0 to Vdr and MBL-1 to ground,and turn on select transistors M0 and M1 so that DBL-0 carries Vdr andDBL-1 carries ground. And to read the left bit of the memory cell “b”,set MBL-1 to Vdr and MBL-0 to ground, and turn on select transistors M0and M1 so that DBL-1 carries Vdr and DBL-0 carries ground.

An exemplary “pattern” for performing drain-side sensing for the rightbit of these other memory cells “b”, “d”, “f”, “h”, “j”, “l ” and “n”may be as follows, with reference to Table 3.

TABLE 3 Voltage Levels on Metal Bitlines Address MBL-0 MBL-1 Selecttransistors “on” read cell “b” Vdr 0 v (ground) M0, M1 read cell “d” Vdr0 v (ground) M2, M3 read cell “f” Vdr 0 v (ground) M4, M5 read cell “h”Vdr 0 v (ground) M6, M7 read cell “j” Vdr 0 v (ground) M8, M9 read cell“l” Vdr 0 v (ground) M10, M11 read cell “n” Vdr 0 v (ground) M12, M13Another Exemplary Read Cycle

In this example, reading is started in the middle of a slice, ratherthan at an isolated end of the slice, as described hereinabove.Generally, in case that the initial read address is in the middle of theslice, the neighbor drain side should be filled up with charge, up tothe drain voltage level, and this should be done prior to the readcycle.

This is presented in the following Table (“Table 4. Reading in themiddle of a Slice”) and is illustrated with respect to FIGS. 14-17.

The left side of Table 4 (below) describes the state of the selecttransistors and MBLs and DBLs state during the read cycle.

The right side of Table 4 (below) defines the state of the select andMBLs during the read cycle pre-pulse address phase used for pre-chargeor discharge.

This exemplary sequence is illustrated with respect to FIGS. 14, 15, 16and 17.

TABLE 4 Reading from the middle of a Slice Add Read Cycle Address logicCell SL DBL MBL add letter FC# t b D S D S 1, 2, 3, 4 Pre-charge 0 5 k12 1, 2, 3, 4 5 10 11 0 1 0, 1, 2, 3, 4, 5 0, 1, 2, Pre-charge 0, 1 3,4, 5 6 m 14 0 0 12 13 2 3

FIG. 14 illustrates a first step, which is a pre-charge phase. Note thedashed-line circle around cell “1”, which indicates that the memory cell“1” is going to be read in next cycle. For example:

-   -   Signal <1>, <2>, <3> and <4> from sl_top turns on select        transistors M2, M4, M6, M8. MBL-0 is set to Vdr, to fill it up        with charge.    -   Select transistors M14, M16, M18 and M20 are also on. MBL-2 is        set to ground (gnd) or allowed to float.    -   There are no signals from sl_nbot. MBL-1 is set to Vdr, to fill        it up with charge.    -   MBL-3 is set to ground (gnd) or allowed to float.

The metal bitlines MBL-0 and MBL-1 are pre-charged.

FIG. 15 illustrates reading cell “k” (logic address 5). For example,

-   -   Signals <1>, <2>, <3>, <4> from sl_top turn on selects        transistor M2, M4, M6, M8. (same as pre-charge step, FIG. 14).        MBL-0 is set to Vdr.    -   The select transistors M14, M16, M18, M20 are also turned on.        MBL-2 is floating.    -   A signal <5> from sl_bot turn on select transistor M11. MBL-1 is        set to gnd. The select transistor M23 may be turned on. MBL-3        remains floating.

With the two transistors M10 and M11 turned on, memory cell “k” can beread, using drain-side sensing. This represents reading of “Address 5”.(Refer to Table 4, logical address “5” corresponds to FC#12, cell “k”).The left diffusion of memory cell “k”, functioning as drain (D), isconnected to metal bitline MBL-0, and the right diffusion of memory cell“k”, functioning as source (S), is connected to metal bitline MBL-1.

Although the transistors M22 and M23 are turned on, the associated MBLsare left floating, so no reading of memory cell “w” is taking place.

Subsequent (every other) cells, such as “m”, “o”, “q”, “s” . . . can beread in the manner described hereinabove.

FIG. 16 illustrates a pre-charge phase prior to commencing reading atAddress 6. Note the dashed-line circle around cell “m”, which indicatesthat the memory cell “m” is going to be read in next cycle. For example,

-   -   Signals <0>, <1>, <2>, <3>, <4>, <5> from sl_top turn on selects        transistor M0, M2, M4, M6, M8, M10. MBL-0 is set to Vdr, filled        up with charge.    -   The select transistors M12, M14, M16, M18, M20, M22 are also        turned on. MBL-2 is floating.    -   Signals <0>, <1>, <2>, <3>, <4>, <5> from sl_bot turn on selects        transistor M1, M3, M5, M7, M9, M11. MBL-1 is set to Vdr, filled        up with charge.    -   The select transistors M13, M15, M17, M19, M21, M23 are also        turned on. MBL-3 is floating.

FIG. 17 illustrates reading cell “m” (address 6, Table 4). For example,

-   -   Signal <0> from sl_top turns on select transistors M0 and M12.        MBL-0 is left floating (cell “a” will not be read). MBL-2 is set        to Vdr, for reading cell “m”.    -   Signal <0> from sl_bot turns on select transistors M1 and M13.        MBL-0 is left floating (cell “a” will not be read). MBL-2 is set        to gnd, for reading cell “m”.

Subsequent (every other) cells, such as “o”, “q”, “s” . . . can be readin the manner described hereinabove.

While a number of exemplary aspects and embodiments have been discussedabove, those of skill in the art will recognize certain modifications,permutations, additions and sub-combinations thereof. It is thereforeintended that the following appended claims and claims hereafterintroduced be interpreted to include all such modifications,permutations, additions and sub-combinations.

What is claimed is:
 1. A non-volatile memory (“NVM”) device comprisingNVM cells in an NVM array; and control logic to cause a circuit topre-charge or discharge one or more bitlines not directly connected withone or more NVM cells being read, wherein pre-charging or dischargingcomprises pre-charging or discharging several global bitlines and localbitlines, and wherein the selection of the global bitlines is doneaccording to whether these global bitlines will interfere with sensingduring a next read cycle.
 2. The device of claim 1, wherein said controllogic further causes reading of said one or more NVM cells to occur inan order which keeps a sensing node of said cells being read facing abitline which has been pre-charged or discharged.
 3. The device of claim1, wherein the pre-charged or discharged global bit lines are selectedaccording to a pre-defined table per each address.
 4. A non-volatilememory (“NVM”) device comprising: a plurality of non-volatile memory(NVM) cells which are arranged in a row of memory cells and which areselectively connected to a first bitline and a second bitline, whereinone of the bitlines is a sensing node; and control logic to cause acircuit to pre-charge or discharge global bitlines and local bitlines,wherein the pre-charged or discharged global bit lines are selected soas to mitigate pipe effect during a next read cycle, and to perform readin an order which keeps a sensing node facing a bitline which wasprecharged or discharged.
 5. The device of claim 4 wherein thepre-charged or discharged global bit lines are selected according to apre-defined table per each address.